1. Field of the Invention
The present invention relates generally to x-ray tubes and more particularly to cathodes or cathode optics for such x-ray tubes.
2. Related Art
Thermionic emission is a very common strategy for obtaining electrons for use in x-ray producing devices. For thermionic electron emission to take place, a source material is heated to high temperature in an evacuated environment in order to impart sufficient energy to bound electrons within the material to liberate them. The energy required is known as the material “work function.” A thermionic electron source is usually made of tungsten in the form of a filament and is often alloyed with other material(s) to reduce work function and/or to improve mechanical properties of the filament under the rigors of high-temperature operation.
A critical aspect of any x-ray device containing a thermionic (or other) electron source is the performance of its electron optics. In a miniature x-ray device, an electric or magnetic field is commonly used to focus the emitted electrons into a beam, directed toward an anode target. The beam of emitted electrons is thereby focused to a desired cross-section at a point in space on or near an anode target, where x-rays are produced upon electron impact.
Cathode optics that produce a beam cross section, or “spot size,” substantially less than the size of the emitter itself are relatively difficult to design and manufacture for use in an inexpensive, miniature x-ray tube. Miniature x-ray tubes impose unique constraints on the optics that do not necessarily exist in the context of their larger counterparts. These constraints include size, energy efficiency, cost and complexity of manufacture and maintenance of tight manufacturing tolerances applied to very small dimensions.
It is challenging to manufacture a miniature x-ray tube that consistently produces a spot size substantially smaller than the emitter. Originating in the filament itself, small variations in filament shape, size and position are problematic, to the extent that they produce relatively large and objectionable changes in the shape, size and position of the electron beam at its intersection with the anode target. The objectionable observable effect to the end-user application is a corresponding variation in the shape, size, position and intensity of the x-ray emissions from the miniature x-ray device.
A common beam-shaping cathode optic used in conjunction with helical tungsten filament emitter is a “T-slot.” The T-slot is well known in the art of x-ray tube optic design. It is, however, difficult to create a T-slot optic of the dimensions appropriate for a miniature x-ray device, wherein the filament coil can be substantially smaller than 1 mm in length. As the dimensions of the filament are smaller in a miniature x-ray device than in a larger-scale device, so also must the dimensions of the electron optics in the vicinity of the filament. A typical t-slot on such a scale is difficult to produce. Even when such a device is created, it is difficult to maintain dimensional tolerances, given the aforementioned variation in filament position and shape within the confines of the optic.
A suitable optic is required for meeting the requirements of manufacturability, tolerance, cost and performance in such respects as electron efficiency, spot size and position.